NEW YORK - The driver of a train that derailed in December in New York City, killing four passengers, had an undiagnosed sleep disorder at the time of the accident, the National Transportation Safety Board said on Monday.

William Rockefeller, the engineer at the controls of the Metro-North commuter train that derailed in the Bronx on Dec. 1, suffered from severe sleep apnea, which can cause drowsiness, according to documents published by the safety board on Monday.

The diagnosis came from a doctor specializing in sleep disorders and who examined Rockefeller in December at the safety board's request.

"Being a shift worker might have contributed to the accident," the doctor, who was not identified, wrote in a statement to the board, according to a medical report among the newly published documents.

The disorder, characterized by shallow or interrupted breathing during sleep, often goes undiagnosed, according to the U.S. National Institutes of Health.

Jeffrey Chartier, Rockefeller's lawyer, also confirmed the diagnosis.

"He was fully cooperative with the NTSB and their investigation, including providing his doctors and medical records," Chartier said in a telephone interview.

Rockefeller's condition appeared to have been aggravated by his assignment to an early-morning shift about two weeks before the crash, Chartier said.

The medical report also said Rockefeller had small amounts of a sedating antihistamine in his bloodstream at the time of the accident.

His last routine physical examination for Metro-North was in May 2011, and his last visit to his own doctor was in May 2013, the report said.

Before the accident, doctors had diagnosed Rockefeller as obese and having hypothyroidism, high cholesterol, low testosterone, vitamin D and B12 deficiencies, and mild high-frequency hearing loss, the report said.

He had never been screened or examined for sleep apnea until after the accident, the report said.

Investigators said the seven-car train was traveling nearly three times faster than the speed limit as it approached a curve in the track just outside the Spuyten Duyvil station in the Bronx. Besides the four deaths, more than 70 people were injured.

Rockefeller has said through his representatives that he may have dozed off shortly before the derailment.

The safety board also released transcripts of interviews with Rockefeller and other Metro-North employees on Monday, along with a report by engineers who examined the scene of the accident.